If the quorum requirement is Q, and X votes are cast, and X is less than
Q, add (Q - X) votes to the ballot which rank the default option soly
first, and rank no other options, and continue the ballot as per usual.
I'm not sure if it solves problems or creates more, but it seems to me
the natural way of resolving quorums. Q could be adjusted if this is too
strong or weak.